1. Field of the Invention
This invention relates to devices for automatically focusing an objective optical system by detecting its focus state on an object and, more particularly, to photographic apparatus or observation apparatus having an optical element releasably attached in the optical path of an objective optical system to change the focal length of the objective optical system with a change of the speed of focus adjustment suited to the timing of focus detection.
2. Description of the Related Art
Recently, the automatic focus adjustment has experienced to wide use in single lens reflex cameras. Compared with the lens-shutter camera with auto-focus capability, a far higher accuracy of focus adjustment is demanded for the single lens reflex camera. Hence, in most cases, the image sharpness is detected repeatedly in certain time intervals during the focusing movement of the photographic lens, and the adjusted amount of movement of the lens is corrected based on the result of the recycled detection.
The recycle period depends mainly on the time during which the detection result is reached, specifically speaking, the sum of the information integration time of the detection sensor, the computing time of the sensor signal and the time of discriminating between the in-focus and out-of-focus states, determining the defocus direction and calculating the amount of defocus. The computing and signal processing time is almost constant for the constant brightness of the object. When, for example, EV=11, it takes about 140 ms.
Meanwhile, of known focus detecting methods, even the one employing the phase difference measuring (correlation) technique has a facility of detecting the defocus amount virtually limited at the upper side, though differing with different types of the detection system, ranging from 10 to 30 m/m. To allow for larger defocus amounts, therefore, a process must be provided in which the measurement is recycled a number of times to approach the true value.
With this, it is of great importance that the speed of focus adjustment be suited to the timing of repetition of the focus detection. If the speed of focus adjustment is too fast, the object image plane of sharp focus will overrun past a prescribed focal plane during the time between the previous and present cycles of detection. In this case, the photographic lens after having once been moved forward has to be returned backward. In fact, it takes a longer time for the lens to reach the in-focus position.
By the way, one of the merits of the single lens reflex camera is in using a wide variety of interchangeable lenses of different focal length selectively. Aside from these lenses, each complete in itself, attachment lenses are also used in combination with an interchangeable lens, including extenders for enlarging the focal length of the master or interchangeable lens, reducers for contracting the focal length of the basic or interchangeable lens, and closeup lenses for shortening the minimum object distance. For telephoto photography, the extender is often used. But the combined lens, though being driven to move at the same apparent speed, gives rise to a phenomenon that the distance the sharp image plane moves in one cycle of detection is increased by about the square of the rate of enlargement of the focal length even at a minimum or when the given detected position is close to the in-focus position.
Therefore, determination of the speed of focusing movement of the lens must be made in such a way that the focus detection device can sufficiently cope with it not only when the lens is used in itself, but also when in combination with the extender. If the speed when in use with the extender is taken for granted, a far greater proportion of photographic situations (than those which necessitate the use of extenders) are inconvenient because the speed of focus adjustment is unduly slow. Particularly, telephoto lenses are apt to increase the defocus amount. From the standpoint of good manageability, therefore, there has been a demand for increasing the speed of focusing movement of the lens as far as possible. For this reason, if the speed is taken at too slow a value, an unsatisfactory performance results with a view to the auto-focus camera.
When the reducer is used, on the other hand, for the constant apparent speed of focusing movement of the lens, the actual defocusing speed of the object image is slowed down by the square of the rate of contraction of the focal length. Therefore, ample room is created for a further minimization of the period of each cycle of operation of the focus detecting device. Of course, the original length of the period is acceptable. Yet, there is much room for improvement of the manageability of the camera. Also when in use with the closeup lens, the same holds. Note that techniques close to the foregoing are disclosed in U.S. patent applications Ser. Nos. 688,355, 829,792 and 913,343.